2013 October 8

First blogging swag

One hears about bloggers making money off their blogs from advertising (I get nothing from the ads that WordPress.com puts on my blog—it just pays for the free blogging service). One also hears about bloggers getting sent products or books to review on their sites, as a form of cheap advertising. I’ve reviewed a number of things on my blog, but they’ve always been things I’ve bought retail, not promotional items.

Yesterday I got my first free swag. Bitscope liked my use of their Pocket Analyzer USB oscilloscope in my circuits blog posts, and are supposedly integrating some of my suggestions into their next software release. They offered me a chance to use and review a pre-production version of a new product: a front-end for the Pocket Analyzer that gives it the ability to do true differential inputs and AC coupling, reducing some of the limitations of the DC-coupling and having the analog ground be the same as the USB ground.

I agreed to this on 2013 July 29, and they supposedly sent me one of the devices. I never got it, though, thanks to the combination of the Australian and US post offices (perhaps it is still on a boat circling the Pacific Ocean). Had I gotten the device over the summer, I would have probably spent a week playing with it, reporting the results on my blog. Now that the Fall quarter is in full swing, I’ll have much less time, what with teaching classes, grading programs, supervising grad students, and helping write grant proposals. I’ll probably be only able to put in an hour or two a week on it (I’ve already spent far more than that today on this blog post, and I’ve still not done any meaningful testing).

Bitscope asked me last week when I was going to post something, and I told them that I was still waiting to get something. So they sent me another one via Fed Ex, which came quite promptly:

Date/Time

Activity

Location

–

10/04/2013 – Friday

1:26 am

Shipment information sent to FedEx

4:49 pm

Picked up

ALEXANDRIA AU

6:02 pm

Left FedEx origin facility

ALEXANDRIA AU

–

10/05/2013 – Saturday

6:07 am

In transit

ALEXANDRIA AU

–

10/06/2013 – Sunday

5:09 am

In transit

LOS ANGELES, CA

10:25 am

Arrived at FedEx location

MEMPHIS, TN

1:29 pm

International shipment release – Import

MEMPHIS, TN

3:27 pm

Departed FedEx location

MEMPHIS, TN

5:26 pm

Arrived at FedEx location

OAKLAND, CA

–

10/07/2013 – Monday

4:50 am

Departed FedEx location

OAKLAND, CA

7:49 am

At local FedEx facility

SOQUEL, CA

8:37 am

On FedEx vehicle for delivery

SOQUEL, CA

9:18 am

Delivered

SANTA CRUZ, CA

I found the routing a little strange (Los Angeles->Memphis->Oakland), but I can’t fault the delivery speed. Within the US, I’ve always had very good service from the US Post Office, with prompt, low-cost delivery, but deliveries across the Pacific can be a bit iffy. It generally takes 2–3 weeks to get something from China through the Post Office, longer from India, and my success rate from Australia is only 50% (small sample, though). It may be worth paying extra for more reliable shipping from Australia and India.

The probe PC board is about 6.2cm long and 2cm wide (8cm long with the connectors on the ends).

The back of the probe has a connector with 3 twisted pairs that go to the Pocket Analyzer, and two jumper positions for shorting plugs that change the gain of the preamplifiers in the differential probes. The label on the probe says 2× without the jumper and 20× with the jumper, but I was told by email 0.5× and 5×, so I’ll need to test the gain for myself. There is no documentation yet, since this is a pre-production prototype. The jumpers for gain switches are a bit of a kluge, but I expect that sort of cost cutting on a USB oscilloscope.

The other end of the short (~14cm) cable from the probe plugs into the leftmost pins of the Pocket Analyzer, connecting to 5v, 3.3v, A, B, and 2 Gnd pins. There are two blank positions on the left end of the connector, corresponding to the space at the end of the connector on the Pocket Analyzer. I suspect that the intent is to use this as a key to reduce the probability of plugging the probe in wrong. It probably won’t help much, as there is nothing stopping anyone from plugging it in upside down at the wrong end of the connector—the Pocket Analyzer uses a symmetric connector that has no keying.

On the component side, we can see that there are are 4 main ICs (probably op amps or instrumentation amps, but I can’t read the labels through the heat-shrink tubing) and a number of discrete components. I’m assuming that the heat shrink tubing is a pre-production measure, and that they’ll have a classier-looking case once they go into production. (I’ve been recommending heat-shrink wrapping for the LEDs in the light gloves my son in designing, because I think that classier cases for the LEDs will be difficult for them to get made in tiny quantities, but I suspect that BitScope is looking at a larger production run than the first run for the light gloves).

Taking a closeup photo of the input end of the probe with my camera lets me see the labels on the ICs. They seem to be MCP6292 E/SN chips (about 67¢ each from DigiKey in 100s). Those are dual rail-to-rail op amps with a 10MHz gain-bandwidth product (7v/µs slew rate), with a 2.4v–6v single-sided power supply. You can also see that the surface mount components have been hand-soldered, by someone who is worse at SMD soldering than me (that’s pretty bad). It looks to me like one of the components (a resistor?) has been shorted by a bad solder bridge. I assume they did some testing before sending me the probe, so I hope that this is an optical illusion, not a real short.

I’ve not heard back from the BitScope folks yet what voltage range is acceptable on the inputs, so I’ll have to be rather cautious at first—the op amps don’t want inputs more than 1v outside the op-amp power rails, and I don’t know what voltage dividers there are before the op amp.

I suspect that the bandwidth for the probe is around 5MHz in the low-gain setting, and 1MHz in the high-gain setting—I’ll probably have to take the setup into one of the student labs at the university to test this, though, as I don’t have much to use for comparison at home. It may be difficult to measure the bandwidth when the probe is plugged into the Pocket Analyzer, since the Pocket Analyzer itself has limited bandwidth. I can’t get it to sample faster than 5MHz, which gives an upper limit of 2.5MHz for the bandwidth. The online documentation for the BitScope claims 20M samples/sec is possible, but if I try to set that, the software claims that there is an access violation and does not change the setting.

I think that the noise floor of the differential probe will also be set by the rather high noise in the Pocket Analyzer. The high-gain setting of the differential probe will allow somewhat better signal-to-noise ratios, as I doubt that the differential probe is anywhere near as noisy as the Pocket Analyzer, which seems to have ±3LSB on an 8-bit DAC (about ±1% of full scale). I wonder whether BitScope will try to take advantage of some of the newer processors (like the Freescale Kinetis L family) that have higher resolution ADC with probably adequate sampling rates.